Our work has involved studying relationships between heart period and blood pressure variability. In this study, we wish to study these effects with cardiac transplants. Subjects will be studied in two sessions, at 3 and 12 months post surgery. Two control groups will be used: normal, healthy subjects and renal transplant patients. Both groups will be matched by age and gender to the transplant recipients. Normals will be studied twice, at the same intervals as the cardiac transplant patients. Renal transplant patients will be studied only once, at 3 months post-surgery. In this psychophysiology study, subjects will complete psychosocial questionnaires and then will be instrumented for ECG, impedance cardiography, noninvasive beat-to-beat blood pressure, and respiration. In the supine position on a tilt table, after a 30 min adaption period, data will be recorded during the following periods: baseline (5 min), mental arithmetic (5 min), recovery (5 min) , arithmetic (5 min), recovery (5 min), Stroop (5 min), recovery (12 min), cold pressor (2 min), recovery (12 min), 70o headup upright tilt (10 min). After this, the tilt table will be returned to horizontal and the study will conclude. Through a catheter placed in forearm vein, blood samples (8 ml each) will be collected 5 times during the protocol.